The term “data unit” herein denotes any data processing device, such as a computer or a personal computer, including workstations or other data terminal equipment (DTE) with an interface for connection to any wired communication network, such as a Local Area Network (LAN).
Transmission lines over which digital signals are transmitted must be properly terminated in order to prevent overshoot, undershoot and reflections. These effects, when caused by impedance mismatch, become more pronounced as the length of the conductor increases, and limit the rate at which data can be transmitted over a transmission line. The transmission line can be a trace on an integrated circuit, a trace on a board, or a wire in a cable. The impedance of both the source and load should be matched to the characteristic impedance of the transmission line. Since the output impedance of a transmitter and the input impedance of a receiver generally differ from the characteristic impedance of a transmission line interconnecting the transmitter and the receiver in a point-to-point configuration, it is necessary to alter the existing impedance differently at the source and load ends of the transmission line.
Wire-based communication networks commonly employ terminations in order to avoid reflections. An example of termination within a network is shown in FIG. 1. A shared wired network 10 is based on a two-wire transmission line having wires 15a and 15b. In the following description, reference will be made to “transmission line 15a and 15b”, it being understood that the reference numerals actually refer to the wires forming the transmission line. For example, network 10 may be an EIA/TTA-485 standard type, wherein transmission line 15a and 15b consists of a single twisted pair, or an Ethernet IEEE802.3 standard 10Base2 or 10Base5, wherein transmission line 15a and 15b is a coaxial cable. In general, the term ‘transmission line’herein denotes any electrically-conductive media capable of carrying electrical current and voltages, and transporting electromagnetic signals, including without limitation wires, cables, and PCB traces. Differential line drivers 11a and 11b are used in order to transmit signals to the transmission line, while line-receivers 12a and 12b are used to receive signals carried over transmission line 15a and 15b. Data unit 16a is a “transmit only” unit, which transmits data to the transmission line via line driver 11a, and data unit 16b is a “receive only” unit that receives data from the transmission line via line receiver 12a. Data unit 16c can both receive data from and transmit data to the transmission line 15a and 15b via line diver 11b and line receiver 12b, forming a transceiver 14. Of course, additional units can be connected to shared transmission lines, each such units employing a line receiver, a line driver, or both. In order to allow for proper operation of network 10, terminators 13a and 13b are commonly installed and connected to both ends of transmission line 15a and 15b. In order to function properly, terminators 13a and 13b should be equal in impedance to the characteristic impedance of transmission line 15a and 15b. Similarly, such terminations are employed in both ends of a point-to-point connection.
The need for termination is a major drawback in building a network. First, the transmission line ends must be identified and accessed, which may not be simple in the case of existing wiring. Additionally, terminator installation requires both labor and materials, and there is also the issue of additional equipment required to configure a network. Furthermore, for proper operation, the termination type, topology and values are mainly based on the transmission line characteristics, which may be unknown and/or inconsistent, and may vary from cable to cable or from location to location.
An additional drawback of network 10 relates to being a multi-point shared transmission line network. In a Time Domain Multiplexing (TDM) scheme, only a single driver can transmit over the transmission line during any time interval, rendering other units as receive-only during that time interval. This limits the total volume of data that can be transported over a specified period. In order to allow multiple data transport over this shared transmission line, it is necessary to allow multiple transmitters and receiver to use the transmission line simultaneously.
One common method for such multiple transmissions over shared transmission line employs the Frequency Domain Multiplexing (FDM) scheme, wherein each transmitter uses a different dedicated portion of the transmission line's available spectrum. Such a solution, however, requires complex and expensive circuitry.
Another method for enabling multiple transmissions is shown in FIG. 2, and involves splitting the transmission line into distinct segments. A network 20 is shown in part, wherein the transmission line is split into two distinct portions, one of which is identified as transmission line segment 15a and 15b (as in FIG. 1), while the other portion is identified as a transmission line segment 15c and 15d. Transmission line segment 15a and 15b is used for full duplex communication using line drivers 11a2 and 11b1, located at respective ends of transmission line segment 15a and 15b. Similarly, line receivers 12b and 12a2 as well as terminators (not shown) are installed at the respective ends of transmission line segment 15a and 15b. Line driver 11a2 and line receiver 12a2 are both part of a unit 21a, which is connected at one end of transmission line segment 15a and 15b. Similarly, transmission line segment 15c and 15d is coupled to line drivers 11c1 and 11b2, as well as to line receivers 12c1 and 12b2. Line driver 11c1 and line receiver 12c1 are both part of a unit 21c, connected at one end of transmission line segment 15c and 15d. Line drivers 11b2 and 11b1, as well as line receivers 12b1 and 12b2 are all part of a unit 21b, connected to transmission line segment 15a and 15b, and to transmission line segment 15c and 15d. These two distinct transmission line segments as well as their related drivers/receivers are coupled by a logic block 22, which is part of unit 21b. In certain prior art configurations, the logic block is either omitted or acts as transparent connection. In such case, unit 21b serves as a repeater. In other configurations, logic block 22 processes the data streams flowing through unit 21b. 
Network 20 offers two major advantages over network 10 as shown in FIG. 1. First, each transmission line segment of network 20 is independent, allowing two communication links to operate simultaneously. Hence, line driver 11a2 of unit 21a can transmit data over transmission line segment 15a and 15b, to be received by line receiver 12b1 of unit 21b. Simultaneously, and without any interference, line driver 11c1 of unit 21c can transmit data over transmission line segment 15c and 15d to be received by line receiver 12b2 of unit 21b. 
Yet another advantage of network 20 is that of having point-to-point communication segments. As is well known in the art, point-to-point topology is a highly favored configuration in wired communication, enabling robust, high bandwidth communications with law-cost, simple circuitry.
Principles of the above description are demonstrated by the evolution of the Ethernet Local Area Network (LAN) as specified in the IEEE802.3 standard, wherein shared transmission line systems based on coaxial cable 10Base2 and 10Base5 were upgraded towards 10BaseT and 10BaseTX based networks, both built around point-to-point segments.
However, network 20 also exhibits a major disadvantage in comparison to network 10. As shown in FIG. 1, network 10 uses a continuous uninterrupted transmission line. In contrast, the wiring of network 20 must be cut at several points throughout the network, wherein units 21 are simply connected. In the case of existing transmission lines (such as in-wall telephone wiring), cutting into the network may be complex, expensive, and labor-intensive.
There is thus a widely recognized need for, and it would be highly advantageous to have, a means for implementing a generic termination that is not transmission line-dependent, and which therefore would not need to be changed when the transmission line characteristics change. There is also a widely recognized need for a means for simultaneous multiple use of a single wiring infrastructure, and for employing a point-to-point connection scheme, without modifying such existing wiring. These goals are addressed by the present invention.